Manage Add-Ons button is where you can add these following services below on top of your base plan.
1000 Listings Exceeding Limit
Additional Source Store
One-Time Setup Amazon or Walmart
Image Background Remover
Click on Manage Add-Ons button
Select the plan which you are currently subscribed on.
Click Edit Subscription
Click Add Addons button
Select the addons you want to apply in your account and hit Add button.
You can view your updated Billing Invoices following this link below.
Access your Billing Invoices